It's a 147-pound showdown, which was made official after yesterday's weigh-in event (see full results here), with Mayweather's WBC and WBA (super) titles on the line against Pacquiao's WBO belt. If Pacquiao defeats Mayweather, he would become the first man in boxing history to become lineal champion in five different weight classes.
That means, throughout his illustrious career, the eight-division champion was at some point the best Flyweight, Featherweight, Junior Lightweight, Junior Welterweight and Welterweight in the world without question.
Mayweather, meanwhile, is a five-division champion in his own right, boasting an unblemished record (47-0) and a reputation as the best boxer of his generation, which includes Pacquiao.